In recent years, nanometer-size wires (hereinafter, also simply referred to as “nanowires”) formed of metal, such as gold and silver, have attracted attention as one of the nanoscale materials. The nanowires are expected to show physical and chemical properties (electrical conductivity, thermal conductivity, light emitting property, and catalytic activity, for example) that are not observed in conventional materials, based on their microscopic size and high aspect ratio, etc. Methods for producing the nanowires are disclosed in, for example, JP 2004-263318 A (Document 1), JP 2004-223693 A (Document 2), JP 2002-67000 A (Document 3), “Formation of nanometer-size wire by irradiating silver colloid with a laser beam”, Takeshi Tsuji and Masaharu Tsuji, Radiation chemistry, Japanese Society of Radiation Chemistry, year 2003, No. 75, page 31 to 34 (Document 4), “Takeshi Tsuji Raisha Shudan” (Takeshi Tsuji & Laser Boys), an Internet site, Takeshi Tsuji and Masaharu Tsuji from Masaharu Tsuji Laboratory, Heterogeneous Integrated Materials, Institute for Materials Chemistry and Engineering, Kyushu University, URL “http://133.5.181.45/TKSTSUJI/”, searched as of Jun. 19, 2006” (Document 5), and “5. Shape change of spherical silver nanoparticles produced by underwater laser ablation with laser beam irradiation (1)”, an Internet site by Takeshi Tsuji and Masaharu Tsuji from Masaharu Tsuji Laboratory, Heterogeneous Integrated Materials, Institute for Materials Chemistry and Engineering, Kyushu University, URL “http://133.5.181.45/TKSTSUJI/researches/photoconv_wire.htm”, searched as of Jun. 19, 2006” (Document 6).
Document 1 discloses a method for forming a copper nanorod or a copper nanowire by heating metal copper and a molybdenum substrate coated with a carbon thin film at a temperature of 800 to 850° C. in vacuum.
Document 2 discloses a method for forming continuously a metal nanowire by applying a voltage or current from a probe tip to a surface of a precursor prepared for producing the metal nanowire, and drawing the metal nanowire at the probe tip. As the precursor for producing the metal nanowire, Document 2 exemplifies a material formed of an inorganic compound with high ion conductivity, on which ions of at least one metal selected from gold, platinum, silver, copper, and palladium are carried. It describes that such a material can be formed by, for example, bringing a powder of sodium-superionic-conductor-type compound or B-type alumina, into contact with an aqueous solution or a molten salt each containing the ions of the at least one metal. A nanowire of the at least one metal can be formed from the precursor.
Document 3 discloses a method for forming a metal nanowire and/or a metal nanoparticle by irradiating a metal ion carrier (for example, Ag2.3Na0.2Zr2Si1.5 P1.5O12. See Production Example 1 of the document) with an electron beam. A silver nanowire can be formed from this carrier.
Document 4 discloses a method in which colloidal silver fine particles with a particle diameter of approximately several nanometers to 100 nm are formed in water by laser ablation, and the colloidal silver fine particles thus formed are irradiated with a nanosecond laser to be fused to each other to form a silver nanowire.
Documents 5 and 6 disclose nanowires formed by irradiating, with an Nd:YAG laser, colloidal silver fine particles (average particle diameter 18 nm) produced by underwater laser ablation as in Document 4. FIGS. 18 and 19 show observation images of the nanowires disclosed in Documents 5 and 6.
In the method of Documents 1, a vacuum and high temperature environment is needed in order to evaporate the metal copper and deposit it on the molybdenum substrate.
The method of Document 2 requires preparation of the precursor for producing the nanowire as a starting material. Also, it is desirable to set the nanowire formation atmosphere to vacuum (8×10−2 Pa or lower) and to a high temperature (100 to 500° C.).
The method of Document 3 requires formation of the metal ion carrier as a starting material. Also, since the nanowire is formed while being in contact with the surface of the metal ion carrier, it is necessary to separate the nanowire from the carrier by a mechanical technique, such as centrifugal separation, in order to obtain the nanowire.
In these conventional methods including the methods of Documents 4 to 6, evaporated metal is deposited on a substrate (Documents 1), metal is deposited by reducing metal ions (Documents 2 and 3), or fine particles with nanometer-size particle diameters are fused to each other (Documents 4 to 6) in order to form nanowires. Therefore, in the case of forming a plurality of nanowires, it is difficult to control their size and shape, leading to a large variation among the resultant nanowires.